A conventional liquid-sealed diaphragm type pressure sensor includes a case having a recess on one side thereof, a sensor chip contained in the recess for pressure detection, an oil filled in the recess and a metal diaphragm for sealing the recess (e.g. U.S. Pat. No. 5,595,939, which is corresponding to JP-A-7-243926).
In this conventional pressure sensor, the recess is filled with the oil, which is used as a pressure transmitting medium and is sealed by the metal diaphragm, so that this oil filled space constitutes a pressure detecting chamber. The sensor chip is contained within the pressure detection chamber.
When the pressure is applied to the metal diaphragm, the diaphragm deforms and the pressure is transmitted to the sensor chip via the oil. The sensor chip outputs a signal of a corresponding level in accordance with the applied pressure so that the pressure is detected.
However, when more than several hundreds of high voltage is applied to this pressure sensor, the same high voltage is also applied to the metal diaphragm. Therefore, the oil is polarized and the sensor chip within the oil is charged.
In the conventional pressure sensor, an integrated sensor chip having a stress gauge and a processing circuit for processing (amplifying or controlling) signals is used. Therefore, when the sensor chip is charged by the oil, the processing circuit for processing the signals is also charged. As a result, an error output is caused from the sensor due to an abnormal operation of the processing circuit.